Wireless-access telecommunications systems are well known in the art. They provide over-the-air (e.g., radio wave, infrared) connections between user communication terminals and a communications switching and transport network such as the telephone network. An illustrative example thereof are cellular radio-telephone systems.
In cellular radio-telephone systems, a plurality of radio cells, also referred to as base stations, are dispersed through a geographical area and each provides radio-telephone service to radio-telephones in its vicinity, referred to as a cell zone. The cells are conventionally connected to the public telephone network through a circuit-switched communications network known in the art collectively as a Mobile Telephone Switching Office (MTSO) or Mobile Switching Center (MSC). When a mobile radio-telephone crosses from one cell zone to another, its servicing is transferred from the cell serving the one cell zone to the cell serving the other cell zone through a process known as a "hard handoff". Adjacent cell sites operate at different radio frequencies, so a "hard handoff" involves a change in the radio frequency that is used to service the mobile telephone. This change in turn requires the cellular radio-telephone system to make a second communications connection to the mobile radio-telephone and to simultaneously drop the first connection. This takes time and uses processing capacity and switching fabric resources, thereby having a negative impact on the system's call-carrying capacity.
Mobile telephony is very popular, and the number of mobile radio-telephones is growing. This results in congestion of the presently-allocated radio-frequency spectrum and a need to more efficiently use that radio-frequency spectrum. The conventional mobile radio-telephony technique, known as frequency-division multiple-access (FDMA), tries to maximize capacity by splitting available bandwidth into separate channels in the frequency domain (e.g., into 30 KHz channels). But the radio-frequency spectrum that is allocated to mobile radio-telephone service is limited to 60 MHz.
A capacity-expanding technique, known as time-division multiple-access (TDMA) is known in the art and is a subject of technical standardization. It is a digital radio technique that splits each 30 KHz channel frequency into a plurality of time slots, each one or more of which can then act as a separate channel. The handoff procedure is similar to that used for conventional mobile radio-telephony, so the TDMA technique can in many instances be handled through conventionally-structured radio-telephone systems with only a change-out of the radio, i.e., the radio-frequency transmission and reception, equipment. But it only increases total system capacity approximately three-fold in mobile applications, which may not be adequate in many congested areas where cellular communications traffic is very high.
An alternative capacity-expanding technique, known as code-division multiple-access (CDMA) has been proposed. It is a dynamic transmission-power control and digital direct-sequence spread-spectrum technique that allows reuse of the same radio-frequency spectrum in adjacent cells. It yields up to approximately a twenty-fold increase in capacity over conventional FDMA systems. Mobile telephones in a CDMA cellular radio-telephone system may undergo "hard handoff" between cells. But, due to the frequency reuse between adjacent cells, a mobile radio-telephone that is crossing from one cell zone to another may sometimes find itself communicating with two cells on the same radio-channel at the same time, a situation known as "soft handoff". A whole sequence of "soft handoffs" may occur as a mobile radio-telephone moves through a series of cells.
Handling of CDMA call capacity and "soft handoff" is not easily accomplished in a conventional mobile radio-telephone system having the conventional FDMA architecture. This is due in large measure to the fact that there are typically many more "soft handoffs" in a typical CDMA system than there are "hard handoffs" in a conventional system and the "soft handoffs" are typically of longer duration than "hard handoffs", and so the demands placed by "soft handoffs" on system resources and processing and switching facilities are more extensive and acute. Handling of "soft handoff" additionally requires, inter alia: routing of the duplicate communications received from one mobile telephone at the two cells to a common call-processing point in the system, for selection in real time of one and discarding of the other duplicate communication; duplication of return communications and routing thereof to the two cells; and coordination of the operations of the two cells so that they transmit the duplicate return communications to the mobile telephone at the same time. Conceivable ways of meeting these requirements in conventionally-architected radio-telephone systems appear to be awkward, inefficient, complex, and expensive.
Furthermore, since each radio at a cell typically requires a unique trunk connection to the telephone network, handling off a call from one radio to another radio requires the mobile-telephone switching fabric to be reconfigured to connect the new radio and trunk to the original network trunk connection. In conventional systems, the total system capacity is a function of the amount of initial radio-to-network trunk connections the system can handle and the amount of reconfiguration (i.e. handoff) the system must perform. The reconfiguration requires intervention of the system control structures, and the length of time required for reconfiguring these trunks increases the complexity of these system control structures. CDMA systems require establishment of a second radio connection for "soft handoff" at rates faster than those needed for traditional handoffs, thereby taxing or exceeding the processing and reconfiguration capabilities of systems of conventional design.